The present invention relates to the recent increase in the research, development, manufacture and use of electronic devices made from wide-bandgap semiconductors, specifically including silicon carbide (SiC) and Group III nitrides (i.e. Group III of the Periodic Table: B, Al, Ga, In, Tl) such as gallium nitride (GaN). Both of these materials have generated such interest for several reasons. Silicon carbide is an attractive candidate material for semiconductor applications because of its wide bandgap (2.99 eV for .alpha.-SiC at 300K) and its other exceptional electronic, physical, thermal and chemical properties.
Gallium nitride, although not sharing all of the same physical properties as silicon carbide offers the electronic advantage of being a wide-bandgap (3.36 eV at 300K) direct-transition emitter. Stated somewhat differently, both silicon carbide and gallium nitride are ideal candidate materials for producing light emitting diodes (LEDs) that because of their wide bandgaps, are capable of emitting at higher energies. In terms of the characteristics of light, higher energy represents higher frequencies and longer wavelengths. In particular, gallium nitride and silicon carbide have bandgaps sufficiently wide to allow them to emit light in the blue portion of the visible spectrum (i.e. wavelengths of between about 455 and 492 nanometers, nm), a color that cannot be directly produced by most other semiconductor materials. A thorough discussion of optoelectronic devices, and their design, the theory behind their operation, is set forth in Sze, Physics of Semiconductor Devices, (1981), and particularly in Chapter 12, pages 681-742, with related discussions of photodetectors in Chapter 13 (page 743) and solar cells in Chapter 14 (page 790). Such background and theory will not be discussed further herein other than as necessary to describe the present invention.
In brief, however, silicon carbide is an indirect emitter, which means that a portion of the energy generated by each transition is generated as vibrational energy rather than as emitted light. In comparison, gallium nitride is a direct emitter in which all of the energy generated by a transition is emitted as light. Thus, at any given current input, gallium nitride offers the possibility for more efficient LEDs, than does silicon carbide. To date, however, gallium nitride has not been produced in bulk crystal form, and thus in order to form an LED or other optoelectronic device from gallium nitride, epitaxial layers of gallium nitride must be formed on some suitable substrate material.
Conventionally, sapphire has been the preferred substrate material for gallium nitride because of its physical properties and because of the generally satisfactory crystal lattice match between gallium nitride and sapphire (Al.sub.2 O.sub.3). Sapphire cannot be made electronically conductive, however, and thus the physical geometry of LEDs formed from gallium nitride epitaxial layers on sapphire substrates are typically of the "same side" variety rather than the generally more preferred "vertical" LED geometry. As used herein, the term "vertical" refers to an LED in which the ohmic contacts can be placed on opposite faces of the device rather than on a common face.
Accordingly, and in addition to its own advantageous electronic properties, silicon carbide provides an excellent substrate material for gallium nitride and other Group III nitride devices. Accordingly, many recent advances in the production of blue LEDs have been based upon a combination of such gallium nitride epitaxial layers on silicon carbide substrates.
Although the manufacture of such GaN-SiC devices has progressed rapidly, epitaxial growth of such materials such as gallium nitride on silicon carbide continues to represent a complex process, and one in which a substantial proportion of attempts produce device precursors that are unsatisfactory for one or more reasons.
More particularly, a GaN on SiC LED typically consists of an SiC substrate with a back ohmic contact, one or more buffer layers on the SiC substrate that provide a crystal lattice transition between the SiC and the GaN, and at least two epitaxial layers of gallium nitride on the buffer layer. The gallium nitride layers include at least one p-type layer and one n-type layer adjacent one another to form the p-n junction of the device. A top ohmic contact is usually made to the top layer of gallium nitride, or in some cases to another material that for some other desired reason forms the top layer of the device.
As well known to those of ordinary skill in this industry, semiconductor substrates are typically sliced from bulk crystals in the form of circular disks, generally referred to as "wafers," upon which various other layers, such as epitaxial layers of GaN, are formed. Because the bulk growth of silicon carbide and the preparation of silicon carbide wafers are both processes which represent significant technical challenge and economic investment, the wafers are in turn quite valuable. If, however, after the gallium nitride epitaxial layers are grown on the SiC wafer, they are found to be defective, or simply unsatisfactory from a desired quality standpoint, the entire wafer becomes a waste product.
Thus, a need exists for removing gallium nitride from silicon carbide in a manner that preserves the silicon carbide wafer. Interestingly enough, the recent success of high quality epitaxial growth of gallium nitride on silicon carbide has exacerbated this problem. Namely, the high quality gallium nitride (and other Group III nitride) epitaxial layers required to produce appropriate LEDs, are similarly much more resistant to the normal techniques (typically wet or dry etching) used to remove unwanted material in conventional semiconductor processes.